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For the road racers: front rocker wheel rate

Did a little experiment with front rocker geometry that might be of interest.

Working on two conditions which emerge from using an off-road chassis for on-road purposes:

1) If you run a low ride-height with the TRX road springs, you need to have the spring collars backed way off... but doing this leaves a bunch of wheel travel effectively free and un-sprung.

2) The geometry of the rocker is off-road optimized; the front rockers have a very long input arm (pushrod side) and a proportionately short output (shock side), something on the order of nearly 2:1 wheel to shock ratio.

This lets the suspension stay nice and compact for off-roading but poses a problem for road use. On road, the wheel movement is far less so it only ever uses a tiny fraction of the shock's length. To account for this narrow range of motion, the shock and spring have to be quite stiff to hold the car up and control the wheel movement.

If you have a look at your average road-racing car pushrod suspension, you'll notice that the input radius of the rocker is much smaller than the shock-end output radius. Completely opposite geometry, let's say 1:2... So a little bit of wheel travel is amplified and spread along a long shock and spring.

Given some constraints - I wanted to still be able to swap a few parts and go from on-road to off-road so had to retain stock pushrods and suspension arms - my execution to increase the shock ratio was to drill a new pushrod hole as inboard on the rocker as possible.

Did some careful measuring of droop and desired ride-height, then drilled a new hole for the pushrod bolt to screw into.

The effective wheel ratio is now pretty much the same as the rear rocker at 1.4:1 ish. As good as we're going to get up front anyway.

This position took out a bunch of un-sprung droop as well as increased the wheel rate by a significant amount. Now the front rocker uses almost all of the shock action, so the damping and spring can both be more compliant. Ride-height is both very low and very well controlled.

I also did this to my Rally, and Love it! I used a set of the HR multi rate rockers with the shortest rear plates on the front rockers and the standard size on the rears. I flipped them upside down so you don't have to pull the whole rocker to remove the shocks/push rods and swapped them side to side. Like I said works great.

Here's a pic of my setup before I pulled the car apart last winter. I'm in the process of resurrecting it now.

Just checked the car, I have the P2s in front and the P1s in the back. I was also mistaken about swapping them side to side, since they are upside down they are on the correct sides in the front, but the rears are swapped.

I also did this to my Rally, and Love it! I used a set of the HR multi rate rockers with the shortest rear plates on the front rockers and the standard size on the rears. I flipped them upside down so you don't have to pull the whole rocker to remove the shocks/push rods and swapped them side to side. Like I said works great.

Here's a pic of my setup before I pulled the car apart last winter. I'm in the process of resurrecting it now.

Yes, that is exactly what I was after... Those plates would have been the only other way to do it but couldn't tell from the photos if it would actually work. Now we know! Looks awesome too...

Did you have to re-cut the upside-down threads for the pushrod and shock screws?

Nope, every thing went right together. They are designed to be screwed in from the bottom to begin with, but that makes removing the shocks or push rods a real pain because you have to remove the entire rocker to get to the screws iff they're on the bottom.

What I did when I bought them was order a set of front rockers and rear rockers and just one set of rear plates. You could just get 2 sets or rears and a set of plates, but they only had one set of rears in stock when I ordered them so I got the fronts too. The plates mount on them the same so it doesn't really matter which you get.

I just re-read this part. So, if I'm using 80wt and 4.3 springs up front, would I even see an increase in handling?

"increase in handling" i suppose depends on what you'd consider that to be... increase in absolute grip? Perhaps not. Increase in stability and compliance, I think so... I run on the street so more compliance for the bumps and texture of the road results in better overall handling.

Since the leverage of the shorter arm changes the ratio, you'll get the same 'stiffness' by using a softer spring and shock fluid.. With the modified rocker, I'm using 3.4 (tan) and 60wt and it's still quite a bit stiffer than my previous setup of the 4.3 (black) on 60wt. I'll likely back the spring out to the stock 2.9 (green) and see how that goes.

Nope, every thing went right together. They are designed to be screwed in from the bottom to begin with, but that makes removing the shocks or push rods a real pain because you have to remove the entire rocker to get to the screws iff they're on the bottom.

What I did when I bought them was order a set of front rockers and rear rockers and just one set of rear plates. You could just get 2 sets or rears and a set of plates, but they only had one set of rears in stock when I ordered them so I got the fronts too. The plates mount on them the same so it doesn't really matter which you get.

Nice, nice... sounds like these are the way to go for a committed road setup.

I suppose the only advantage to modifying the stock rockers would be to have slightly easier way to convert the setup back to long-throw off-road use (just change the pushrod position rather than swap out the whole rocker)... six of one, half-dozen of the other.

Backed the front down to the stock 2.9 (green) springs and it seems to be perfect. Not sure if a 50% increase in wheel rate translates to an effective 50% increase in stiffness, but the 2.9 are just a tad stiffer on the modded rockers than 4.3 are on the standard rears. Appears to handle well, feels like a good even match now.

I tried this but placed the wholes farther from the stock location. WOW was this a big change. I may drill another whole between the two. I didn't change anything so clearly this is a game changer and set-up is completely different. I don't have other springs so I will have to do some changing. I think Diff Viscosity is my first change. I need something to help with wheel spin in corners. Once I tune my changes up I will make a video to show how the car handles.

i'm gonna have to give this a try. see how it affects the handling on my kit. I'm running slash spings since i had a slash before it was turned into a merv. should i keep to the rally springs or can i get away with my slash springs? i have like 3 sets of slash spings and one set of rally springs.

Any of the Slash-length springs will fit but since they're longer they put more pre-load on the suspension which pushes the ride-height up. Doesn't look bad for road use - 6.5-ish mm under the 'steerer' skid - but there's also zero droop which will make the handling skittish. The way I've got em dialed, the stock Rally will run at about 3.5-4mm with a couple mm of droop on this rocker setup. (The collars are about 7 half-turns from the top - the springs just barely tensioned in full droop.)

Stock Slash are the green-striped ones, irrc, which are 1.92lb/in. With the faster rockers, they act as if they're stiffer than that so I'd say go with the lighter-weight Slashes if you have them to allow some droop in (double-blacks, double-tans). Or try the stock Rally springs.

I have Tan Rally springs; Green, tan and black Slash springs; and some MERV tan and black springs. I don't have blue summit springs. I ordered the black rally spings. So if I keep my rocker mod then I should change to a red spring that runs the best. We can always cut down the red springs.

My rocker mod wholes are another two diameters toward the pivot than yours. I should be able to drill another whole without hurtung strength too much.

I have about 1/4 of these for mine various versions of the model. On the Rally or other on-road set-ups I plan to use Rally tan front and Rally black rear and adjust the rocker position to get the front handling I want. I have only tried Rally Tan front and rear only. I think I have Slash Double Green, Revo Black and Tan. So with the rocker change I may start with Revo Black.

No cutting. I just chose a position that was near ride height. The push rods may actually be touching. I need to reposition them for sure.

One thing I also noticed is that on you changes you moved the new whole such that you could get more travel on the shock. You must have had to remove the travel limiters in the shock so that you can get more shock movement. Did you change that? It would be great to be able to use the full travel of the shock with an on-road set-up.

I do have the rally limiters in my shocks and that mod makes a world of difference. On a half dead nimh i use for on the bench servo centering and such it was excellent. i wanna see how it will behave with a lighter more powerful 2s lipo. The chief handling problem I am gonna gun into the the back seems to slide around fairly easy but i'm only beginning the tuning process.

One thing I also noticed is that on you changes you moved the new whole such that you could get more travel on the shock. You must have had to remove the travel limiters in the shock so that you can get more shock movement. Did you change that? It would be great to be able to use the full travel of the shock with an on-road set-up.

Nope, still have limiters in. What this mod does is actually use nearly all of the shock compared to the stock setup where a fraction of the shock was active (particularly if you ran with low ground clearance and the necessary low preload tension to get there).

drilled mine tonight thanks for the tip...also found this on another site

I'd recommend for a base line ON ROAD set up
The 4.3 rate front Rally springs TRA7342
The 3.4 rate rear Rally springs TRA7341
Good set of tires

i have the springs on order now, we race wed night i will post up on how the drilled rockers work

Yup, have been running those same rates (and HPI X pats!) on the street. Keep in mind that the modified rocker changes the effective spring rate by reducing the pushrod leverage so you'll want to run softer springs up front. Fwiw, the stock 2.9 (green) seem a tad stiffer on the modded rockers than the 4.3s do on the standard setup.

I have the RPM A-arms, so I think the push rods will rub if I drill where you did. I'd probably have to drill closer to the stock location and, so I don't think it would make the dramatic difference as you guys. Prototyp, what do you think?

The only reason I got the rpm parts was because I cracked one of the hub carriers bashing one day. So I went ahead and bought all of the arms while getting new hubs. I ran into the binding I described by doing this mod. Good thing I still have the stock a-arms. I'll go try this new set-up at the track today. As far as durability of the RPM stuff, they're definitely more flexible than the stock parts.

For the rocker mod to really make a difference, the pushrod will need to run with very little clearance next to the stock arm. From what I can tell RPMs are a wider shape, so it's likely going to be problem.

Fwiw, I'd be interested in seeing a side-by-side comparison of the RPM against stock to get a sense of how much beefier they are.

Here are a couple of pics of RPMs vs stock front upper arms. There is about 1mm difference in the width of the arm, but the height is the same. I don't have them installed yet, I will try and post some pics when that happens.

Fwiw, my "Rally" started life as a hand me down slash a buddy had used as a dirt oval racer. He used modded push and toe rods when he raced it to lower the ride, and he carved into the stock arms for clearance. I have driven/crashed the car quite a bit, and with a bb for a while and never had a failure of the stock arms even with the top front cut down to almost 1/2 it's thickness.

i got my hr rockers tonight, they do the same job as drilled ones but i got more bling factor now

Cool. Is that just a standard rear set? Or did you have to buy front rockers and separate rear plates? Guess what I'm after is knowing how HR handled the rear rocker's hub offset (if they had made special front vs rear hubs or not).

yes its just the standard rear set...i will get another rear set with tuning plates as soon as i sell some more crawler stuff. i will try the stiffer 3.4 springs in the rear this wed night and see how that does